In a closed-loop optical modulation system, the intensity of a modulated optical output beam may be monitored and an electrical signal which controls the modulation of the beam may be appropriately adjusted in an effort to stabilize the operation of the system. In an optical modulation system employing an acousto-optic modulator, system control is affected by the non-linearity of the transfer function of the modulator itself. That is, an acousto-optic modulator may operate so as to deflect light in proportion to the input power to the modulator. To monitor the output light intensity of the modulator, a photodetector is usually optically coupled, as by way of a beam splitter, to the output beam, and the photodetector generates an output the magnitude of which is proportional to the detected light intensity. Since this detected output is fed back to the modulator as a control signal, it can be seen that there is an inherent square law non-linearity in the modulator control loop, input power to the modulator being proportional to the square of current.
An exemplary configuration of a closed loop acousto-optic modulator is depicted in FIG. 1. The output beam 17 from a laser 16 passes through a suitable acousto-optic modulator (AOM) 15, whereby beam 17 is deflected in accordance with the modulation input power supplied from a suitable modulation source, such as a mixer 14. Mixer 14 modulates the output of an RF oscillator in accordance with an information signal input coupled through signal combining circuit 11, such as a suitable summing amplifier, and subjected to appropriate gain control in gain control circuit 12. The deflected output beam 18 from acousto-optic modulator 15 is separated into an output component 20 and a feed-back or control component 22 by a suitable beam splitter 19. The feed-back component 22 may impinge upon a photodetector 21 which produces an output, the magnitude of which is proportional to the intensity of the light impinging upon the detector. This photodetector output is fed back to signal combining circuit 11 to adjust the magnitude of the signal current supplied to mixer 14, in order that the output light intensity may be controlled in accordance with the input signal in the presence of light beam intensity fluctuations.
Due to the non-linearity in the loop created by the square law relationship between the deflection of beam 17 by acousto-optic modulator 15 and the modulation signal current supplied by mixer 14, it has heretofore not been possible to provide a uniform bandwidth independent of output light intensity level.